Four wheel drive for vehicles can be advantageous in certain circumstances, like when additional traction is needed because of surface conditions, or during cornering or acceleration. Driving all four of the wheels and their associated components, however, is not often required during most driving conditions and it results in increased fuel consumption.
It is preferable that two of the four wheels be selectively engaged only when required at least to avoid the increase in fuel consumption. It is known to directly drive two wheels of a vehicle and then to use a power take off unit to selectively drive the other two wheels. Typically, a shift fork and a sliding clutch, among other components, are used to selectively engage and disengage the rear wheels at the power take off unit.
One embodiment of a prior art shift fork 10 and sliding clutch 12 is depicted in FIG. 1. The shift fork 10 is moved in the axial direction by a linear push rod or piston 14. The shift fork 10 is connected to the sliding clutch 12. The sliding clutch 12 slides on, and rotates with, a source of rotation. In the depicted in embodiment, the sliding clutch 12 is mounted for axial movement on a ring gear 16. The ring gear 16 is driven by a shaft 18.
The clutch 12 has a set of teeth 20 on one of its side surfaces. The shift fork 10 selectively moves the sliding clutch 12, and its teeth 20, axially into and out of engagement with a set of teeth 22 on an adjacent shaft 24. The shaft 24 is connected to a drive shaft 26, such as an axle half shaft. As shown in the figures, the adjacent shaft 24 is concentric about the drive shaft 26.
The above-described system has a number of drawbacks. First, it requires a large amount of space for the sliding clutch 12 to be translated in the axial direction. Second, it requires a relatively large and powerful device to move the entire fork 10 and the entire clutch 12. Third, because the clutch 12 is moved, the shift fork 10 and other components must be robust, and thus heavy, to withstand the repeated loading and unloading. Fourth, the response time for the clutch 12 to be engaged or disengaged is slow often because of the large amount of time needed for the shift fork 10 to axially move the clutch 12 adequately for engagement or disengagement with the adjacent set of teeth 22.